Surveillance cameras must deal with wide variations of lighting -- automatically and continuously. It can be 10,000+ lux during the day and under 1 lux at night. In a few seconds, lighting can drop dramatically, for example, if the sun goes behind a cloud or if a person turns off the lights. As lighting changes, the camera needs to adjust on its own without having a photographer optimize the scene.
Surveillance cameras do this through two means - controlling the size of the iris or the speed of the shutter. Indeed, often cameras can control both simultaneously. In this tutorial, we compare the two and explain how they relate.